mediatek: make use of Acer Predator Connect W6's u-boot environment

mediatek: make use of Acer Predator Connect W6's u-boot environment

In order to prepare OpenWrt support for other Acer W6 devices and to adapt
the procedure to read and set mac addresses which other devices of the same
target are using (instead of needing an additional script and creating an
additional structure in the file system), this commit
- reads device mac addresses from u-boot environment
- avoids the detour via the file system to set the mac addresses
- drops redundant file /lib/preinit/05_extract_factory_data.sh

The idea and the implementation were thankfully taken from PR #16410.

This is the second of four commits into which the original commit was split
to make reviews easier and more targeted.

Signed-off-by: George Oldfort <openwrt@10099.de>
Link: https://github.com/openwrt/openwrt/pull/16861
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>